This invention is generally directed to a system and process for designing positive charge sharing toner compositions, and more specifically the present invention is directed to a process for rapidly charging uncharged toner particles to a positive polarity, which toner particles are useful for causing the development of electrostatographic images, wherein the electrostatic latent image to be developed contains negative charges.
The electrostatographic process, and more specifically, the xerographic process is well known, as documented in several prior art references. In these processes, an electrostatic latent image is developed by applying toner particles thereto using, for example, the cascade development method as described in U.S. Pat. No. 3,618,552, or magnetic brush development as described in U.S. Pat. Nos. 2,874,063 and 3,251,706. In some instances, as indicated herein, it may be desirable in such systems to produce a reverse copy of the original, thus, for example, it may be desired to produce a negative copy from a positive original, or a positive copy from a negative original.
In cascade development, developer material comprising relatively large carrier particles containing oppositely charged finely divided toner particles electrostatically clinging to their surface are conveyed to and rolled or cascaded across the surface bearing the electrostatic latent image. The charged portions of the surface, generally a photoconductive surface, have a charge of the same polarity as the carrier particles, however, such charge is usually much stronger than the charge on the carrier particles. Thus, as the developer mixture cascades or rolls across the image bearing surface, the toner particles are electrostatically attracted from the carrier particles to the charged portions of the image-bearing surface, whereas they are not electrostatically attracted to the uncharged or background portions of the image which they contact. The carrier particles, and unused toner particles are then recycled.
Magnetic brush development involves essentially the same steps as cascade development with the primary exception being that in magnetic brush development, developer material comprising relatively large magnetic carrier particles and finely divided toner particles electrostatically clinging to the surface of the carrier particles are conveyed to and drawn across the surface bearing the electrostatic latent image by a roller containing magnets.
In order to develop a negatively charged electrostatic latent image a toner and carrier combination is selected wherein the toner is triboelectically positive in relation to the carrier, while for the development of a positively charged electrostatic image, a toner and carrier combination is used wherein the toner is triboelectrically negative in relation to the carrier material. In these systems the triboelectric relationship between the toner particles and carrier particles depends on the relative positions of the materials in the triboelectric series. In this series materials are arranged in ascending order of their ability to assume a positive charge, thus each material is positive with respect to any material classified below it in the series, and negative with respect to any material above it in the series. An example of such a triboelectric series is illustrated in FIG. 3.
There is also known the use of certain charge control agents for the purpose of imparting a positive charge to the toner resin. For example, the use of quaternary ammonium salts as charge control agents for electrostatic toner compositions is disclosed in U.S. Pat. No. 3,893,935. According to the disclosure of this patent, certain quaternary ammonium salts when incorporated into toner materials were found to provide a toner composition which exhibited relatively high uniform and stable net toner charge, when mixed with a suitable carrier vehicle. U.S. Pat. No. 4,079,014 contains a similar teaching with the exception that a different charge control agent is used, namely a diazo type compound.
Many of the above described developers have a tendency to lose their positive charge over a period of time. Also, certain charge control additives are incompatible with the thermoplastic resin, causing difficulties in uniformly dispersing or dissolving such materials in the toner composition. Additionally, chemically active charge control agents can adversely effect machine components such as seals, rubber rolls, and the like. Thus, it would be desirable to provide a chemically inert toner charging means.
Furthermore, during the operation of an electrostatographic imaging device, toner particles being consumed must be constantly replenished. The amount of toner added to the developer composition depends on a number of factors including the number of images developed per minute, the percentage area of the image on the paper, the darkness of the image, the size of the toner particles, and the like. Generally, the amount added ranges from about 1 to about 20 percent per minute of the total amount of the toner in the developer. When replenishing the developer material, that is, toner plus carrier, used in commercial electrostatographic devices, the newly added toner does not contain any charge thereon, that is, its charge is neutral. In order for the toner to properly cause development of an electrostatic latent image the newly added toner must be charged to the appropriate level, which could consume a substantial period of time, up to about one hour depending on the materials used and other parameters of the system. This long time delay can adversely affect the system in that the toner particles that possess no charge, or a low charge can be printed out as undesirable background material. Also, uncharged toner particles will in general result in contaminated machines in view of the deposition of such particles on machine parts, thereby eventually causing failure of corotrons, filters, and the like. Such failures are not only costly, but also result in images of low quality.
Accordingly, there is a need for a system, and more particularly a process wherein newly added uncharged toner particles contained in a positively charged developer composition can acquire positive charge at the appropriate level and magnitude over a short period of time, by sharing positive charges with charged toner particles previously present in the developer composition. This is referred to herein as positive charge sharing.